Study of ion-beam-sputtered Mo/Si mirrors for EUV lithography mask: influence of sputtering gas

In the field of microelectronics, Extreme Ultraviolet (EUV) lithography operating at λ=13,5 nm, appears, today, as the most promising future technology. The viability of this next generation technology has to face, however, several technical issues. Among them, the realisation of defect-free lithography masks is certainly one of the most serious issue. These masks, which work as reflective components, are composed of an EUV reflective Mo/Si multi-layer coating deposited on a glass substrate and covered, on the top, by an absorbing stack. Up to now, as far as defect specification is concerned, Ion Beam Sputtering (IBS) deposition has been proved to be the most appropriate technology. Nevertheless, the necessity to meet other specifications like, for instance, maximum EUV reflectance, requires further studies to better understand the way such IBS multi-layer Mo/Si structures are growing. In this paper, an experimental investigation on the deposition of IBS Mo/Si multi-layers is presented. We focused our interest on a comparison between argon and xenon sputtering conditions. After a brief description of deposition process, the mechanical and optical properties of the films are first reported. Then, the microstructures and chemical profiles of Mo/Si multi-layers, deduced from investigations by high-resolution transmission electron microscopy (HRTEM) and energy filtering TEM, are presented. Finally, the Mo/Si mirror characteristics are discussed on the basis of TEM observations and process considerations related to sputtering mechanisms taking place in the IBS deposition tools.